Mobile networks are currently evolving from pure circuit switched networks towards IP based networks, and by that seamlessly integrate into IP (Internet Protocol) based infrastructures that are also used for the Internet, the World Wide Web and the data communication industry.
With this trend mobile networks follow the evolution steps of the wireline networks, where VoIP (Voice over IP) via DSL (Digital Subscriber Line) access or via WLAN (Wireless Local Area Network) access is existing technology today.
Mobile operators that install IMS (IP Multimedia Subsystem) networks and offer IMS services, want to make these services available also to GSM/WCDMA subscribers. Hereto, calls from and to this group of GSM/WCDMA subscribers need to be routed through the IMS network in order to reach the IMS service engine. This concept is called (MS Centralized Services (ICS). The (MS centralized Services work item in 3GPP targets at using the circuit switched access for access to IMS services, see 3GPP TS 23.292, and is complemented by IMS Service Continuity, see 3GPP TS 23.237. Telephony services using the IMS network are also referred to as MMtel (Multimedia Telephony) services.
In parallel to the above trend, the packet core is evolving (Evolved Packet Core, EPC) as part of the Evolved Packet System (EPS), supporting eUTRAN as new radio access network. As part of this discussion, work on Single Radio Voice Call Continuity (SR-VCC) is ongoing in 3GPP SA2 (see 3GPP TS 23.216), enabling to transfer an IMS voice call from EPS to the circuit switched domain and vice versa.
When a user of MMTeI services issues an emergency call via a packet switched access, i.e. an IMS Emergency Call, the emergency call is routed to a P-CSCF (Proxy Call Session Control Function) in the visited network. This P-CSCF detects the emergency call and routes it to the E-CSCF (Emergency CSCF) in the visited network. The E-CSCF is configured to handle emergency call sessions, which typically involves identifying a Public Safety Answering Point (PSAP) or an emergency centre.
Here, it should be noted that the E-CSCF is always in the network where the subscriber is roaming. So if the subscriber is roaming in a visited network, also the E-CSCF is located in the visited IMS network. The E-CSCF either routes it to an IP capable PSAP or via a BGCF (followed by a MGCF etc.) to a PSAP in the circuit switched domain. In this respect, it should be noted that presently a PSAP is typically not implemented with IP capabilities, so that emergency calls are typically routed via the circuit switched domain.
Due to the above concepts of handling emergency sessions and legal obligations, it is necessary for the operator of a visited network to provide an IMS infrastructure for IMS roamers, even if the operator does not offer IMS services for own subscribers.
Accordingly, there exists a need to provide an efficient technique for handling emergency calls established via a packet switched network access.
Further, the simple support of IMS Emergency Calls is not enough in order to fulfill the legal requirements of many countries. It may further be required that an established emergency call is maintained, also when roaming in/out of the IMS capable packet switched radio access. This handover is covered for normal speech calls by the 3GPP features “IMS Service continuity” and “SR-VCC”. In both cases the call is anchored in an IMS application server such as described e.g. in 3GPP TS 23.216 and 3GPP TS 23.237.
Accordingly, there is also a need for providing session continuity of emergency calls established via a packet switched network access, e.g. using IMS services, when the user equipment from which the emergency call issued roams to a circuit switched network access or vice versa.